Chemically-resolved aerosol eddy covariance flux measurements in urban Mexico City during MILAGRO 2006

As part of the MILAGRO 2006 field campaign, the exchange of atmospheric aerosols with the urban land- scape was measured from a tall tower erected in a heav- ily populated neighborhood of Mexico City. Urban submi- cron aerosol fluxes were measured using an eddy covari- ance method with a quadrupole aerosol mass spectrometer during a two week period in March, 2006. Nitrate and am- monium aerosol concentrations were elevated at this loca- tion near the city center compared to measurements at other urban sites. Significant downward fluxes of nitrate aerosol, averaging 0.2 µg m 2 s 1 , were measured during daytime. The urban surface was not a significant source of sulfate aerosols. The measurements also showed that primary or- ganic aerosol fluxes, approximated by hydrocarbon-like or- ganic aerosols (HOA), displayed diurnal patterns similar to CO2 fluxes and anthropogenic urban activities. Overall, 47 % of submicron organic aerosol emissions were HOA, 35 % were oxygenated (OOA) and 18 % were associated with biomass burning (BBOA). Organic aerosol fluxes were bi- directional, but on average HOA fluxes were 0.1 µg m 2 s 1 , OOA fluxes were 0.03 µg m 2 s 1 , and BBOA fluxes were 0.03 µg m 2 s 1 . After accounting for size differences (PM1 vs PM2.5) and using an estimate of the black carbon component, comparison of the flux measurements with the 2006 gridded emissions inventory of Mexico City, showed that the daily-averaged total PM emission rates were essen- tially identical for the emission inventory and the flux mea- surements. However, the emission inventory included dust and metal particulate contributions, which were not included in the flux measurements. As a result, it appears that the in- ventory underestimates overall PM emissions for this loca- tion.